The present invention relates to thermochromic microencapsulated pigments. More particularly, the invention relates to microencapsulated pigments comprising microcapsules containing a composition reversibly undergoing color changes with temperature.
Several proposals have hitherto been disclosed on a composition which begins to change from a colored state to a decolored state upon heating, comes into a completely decolored state at temperatures not lower than a specific temperature, begins to color upon subsequent cooling, and returns to the colored state.
Such compositions can be roughly classified into two types. The first type changes in color at a color change temperature in such a manner that at ordinary temperature the composition exists only in specific one of the two states respectively corresponding to both sides of the color change temperature. Namely, the other state is maintained only during the period in which the heat or cold required for the development of this state is kept being applied, and the composition return to the original ordinary-temperature state upon removal of the heat or cold. This type of composition is hence a thermochromic composition which gives a temperature-color density curve showing that it undergoes color changes with a relatively narrow hysteresis range (xcex94H) (see, for example, U.S. Pat. Nos. 4,028,118 and 4,732,810).
The second type changes in color along routes which differ considerably between the case in which the composition is heated from a temperature on the lower-temperature side of a color change temperature range and in the reverse case in which the composition is cooled from a temperature on the higher-temperature side of the color change temperature range. This type of composition gives a temperature-color density curve showing that it undergoes color changes with a wide hysteresis range (xcex94H) Namely, this type of composition is a thermochromic color-memorizing composition in which the colors respectively formed at a temperature not higher than the lower trigger temperature and at a temperature not lower than the higher trigger temperature can be alternately memorized and retained in the temperature range between the lower trigger temperature and the higher trigger temperature even after the heat or cold required for the color change has been removed (see U.S. Pat. Nos. 4,720,301, 5,879,443, and 5,558,699).
The compositions described above have a color change behavior such as that shown by the temperature-color density curve given in FIG. 1, although they vary in the width of hysteresis range (xcex94H). This color change behavior is ascribable to and specified by a reaction medium (c) which determines the temperatures at which the color reactions of an electron-donating chromatic organic compound (a) with an electron-accepting compound (b) take place.
The compounds effectively usable as the reaction medium (c) are limited in kind. In addition, in systems containing effective compounds, there has been no means for shifting the temperature-color density curve. Namely, it has been difficult to delicately regulate color change temperatures, and the conventional compositions have not always had intended color change properties.
The present inventors made intensive investigations on thermochromic compositions. As a result, they have found that the temperature-color density curve can be delicately modified by incorporating a specific compound as the fourth component. The invention has been completed based on this finding.
An object of the invention is to provide thermochromic microencapsulated pigments in which the color change properties of a conventional three-component system comprising components (a), (b), and (c) have been regulated, as a base, with respect to color change temperature. The pigments of the invention are applicable to a variety of fields including temperature indication, decoration, toys, and teaching material elements.
The invention provides a thermochromic microencapsulated pigment which contains as four essential components (a) an electron-donating chromatic organic compound, (b) an electron-accepting compound, (c) a reaction medium determining the temperatures at which the color reactions of component (a) with component (b) take place, and (d) a color change temperature regulator, the color change temperature regulator (d) comprising one or more compounds selected from esters, alcohols, ketones, acid amides, hydrocarbons, and fatty acids and having a melting point Y (xc2x0 C.) satisfying the relationship (X+30)xc2x0 C.xe2x89xa6Yxe2x89xa6200xc2x0 C., wherein X (xc2x0 C.) is the melting point of component (c), and the four essential components being microencapsulated. In preferred embodiments of the pigment of the invention described above, the color change temperature regulator (d) is selected from aliphatic esters, aliphatic ketones, aliphatic alcohols, aliphatic acid amides, and saturated fatty acids; the color change temperature regulator (d) is contained in an amount of from 0.01 to 0.3 part by weight per part by weight of the reaction medium (c); the microencapsulated pigment has an average particle diameter in the range of from 0.01 to 6 xcexcm; and the pigment gives a temperature-color density curve showing that the pigment undergoes color changes with a hysteresis range (xcex94H) of 5xc2x0 C. or narrower.
The invention further provides a thermochromic color-memorizing microencapsulated pigment which contains as four essential components (a) an electron-donating chromatic organic compound, (b) an electron-accepting compound, (c) a reaction medium determining the temperatures at which the color reactions of component (a) with component (b) take place, and (d) a color change temperature regulator, the color change temperature regulator (d) comprising one or more compounds selected from ethers, esters, ketones, acid amides, and fatty acids and having a melting point Y (xc2x0 C.). satisfying the relationship (X+16)xc2x0 C.xe2x89xa6Yxe2x89xa6(X+100)xc2x0 C., wherein X (xc2x0 C.) is the melting point of component (c), and the four essential components being microencapsulated, and which gives a temperature-color density curve showing that the pigment undergoes color changes with a hysteresis range (xcex94H) having a width of from 5xc2x0 C. to 80xc2x0 C., and in which the colors thereof respectively in a range of temperatures not higher than the lower trigger temperature and in a range of temperatures not lower than the higher trigger temperature are alternately memorized and retained by the pigment in the temperature range between the lower trigger temperature and the higher trigger temperature. The invention furthermore provides a thermochromic color-memorizing microencapsulated pigment which contains as four essential components (a) an electron-donating chromatic organic compound, (b) an electron-accepting compound, (c) a reaction medium determining the temperatures at which the color reactions of component (a) with component (b) take place, and (d) a color change temperature regulator, the color change temperature regulator (d) comprising one or more compounds selected from ethers, esters, and fatty acids and having a melting point Y (xc2x0 C.) satisfying the relationship (Xxe2x88x9210)xc2x0 C.xe2x89xa6Yxe2x89xa6(X+15)xc2x0 C., wherein X (xc2x0 C.) is the melting point of component (c), and the four essential components being microencapsulated, and which gives a temperature-color density curve showing that the pigment undergoes color changes with a hysteresis range (xcex94H) having a width of from 5xc2x0 C. to 80xc2x0 C., and in which the colors thereof respectively in a range of temperatures not higher than the lower trigger temperature and in a range of temperatures not lower than the higher trigger temperature are alternately memorized and retained by the pigment in the temperature range between the lower trigger temperature and the higher trigger temperature.
In a preferred embodiment of the two pigments of the invention described above, the color change temperature regulator (d) is contained in an amount of from 0.01 to 0.3 part by weight per part by weight of the reaction medium (c).